ETA: Dipole / Active Balun "First Light": Oct 8, 2005

Overview: Shown here are the results of a quick-and-dirty field test of the ETA dipole & active balun at a rural location near Blacksburg, VA on October 8, 2005.

Maintainer: Steve Ellingson (Virginia Tech)
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10 Oct 2005 UT 1900. Initial posting.

Summary: Good news is that we detect the Galaxy with useable S/N and we do it in the presence of some truly obnoxious RFI. The bad news is that the matching scheme used in the first active balun has proven to be a bad idea - not only does it actually reduce bandwidth somewhat, but it seems to significantly degrade the noise figure in the process. Although the result is not so bad that we couldn't use this design for ETA, I'm confident both of these warts can be improved upon simply by eliminating the matching section.

Procedure: The measurements were conducted using the first ETA active balun prototype with 1.5 mm-dia stranded copper antenna wire for dipole arms. See a picture of the actual setup below. The active balun output was connected to a 150-ft section of LMR-400. At the other end, the signal was pre-amplified using a Mini-Circuits ZJL-3G amplifier, and then input to a Rhode & Schwartz FSH3 handheld spectrum analyzer. The spectrum analyzer is connected to a laptop via optical RS232, and data is acquired using a C-language program written by the author. Each plot below represents the linear average of 100 sweeps with the spectrum analyzer set to "sample" (as opposed to "autopeak") detection to facilitate meaningful averaging. Results shown are the levels at the input of the spectrum analyzer; to get the levels referenced to the active balun input, subtract 45 dB.

Test Setup: Location is Caldwell Fields, a recreational area in a rural area about 20 minutes drive north and east of Blacksburg, VA. The site is located in a valley between two mountains, but is nevertheless still to close to the I-81 corridor to really get much RFI suppression. The active balun is mounted on a copper pipe and the dipole arms are formed by simply lashing them to the fence as shown. The local time was about 5:30 PM, and Galactic Center was close to it's daily maximum elevation.
5-95 MHz, 300 kHz resolution. The good, the bad, and the ugly! First, note the hump centered on 38 MHz: That's the Galaxy looking pretty much like what we were expecting, as indicated by the good agreement with the "predicted" curve. The prediction is developed from Cane's empirical formula for the intensity of the Galactic background radiation, and is modified to account for the theoretical antenna response and measurements of the active balun and instrumentation responses. So, good agreement here -- especially in the absolute levels -- is pretty strong confirmation. And, of course, there is RFI from the usual sources below 20 MHz and above 70 MHz. Notice the HDTV signal on TV Ch 3 (60-66 MHz): Not sure what is going on with that; most HDTV is currently in UHF. Still, nice to see we can get a nice clean acquisition of the Galactic noise in the presence of all this stuff. Also shown are the predicted active balun noise (more on this below) and the output when the active balun is turned off.
23-53 MHz, 100 kHz resolution. Zooming in now to the region of most interest to ETA. Also now showing the results when the dipole is replaced with a 50-ohm load and a short, compared to the predicted noise from the active part of the active balun. This result is a little disappointing; basically it means that the noise temperature of the balun is significantly higher than the anticipated 340K. We could actually live with this (we would be galactic-noise limited by 6 dB over 15 MHz) but I suspect this can be improved simply by getting rid of the matching network.
37.45-38.35 MHz, 3 kHz resolution. I include this mainly to demonstrate that even here, and as bad as the RFI seems to be in the above spectra, the spectrum nevertheless looks pretty good if you use sufficient spectral resolution.